The objective of this research is to determine the changes of the microbial community composition in the Gulf of Mexico (GoM) coastal sediments when exposed to Macondo MC252 crude oil. Due to the low level of natural seafloor seepage of crude oil, indigenous biodegradative microorganisms in the GoM have evolved to metabolize almost all components of crude oil, ranging from simple aliphatic hydrocarbons to polycyclic aromatic hydrocarbons (PAHs). Therefore we expect that exposure to crude MC252 oil to microbial community in GoM coastal sediments will alter the community composition, selecting the oil-tolerant and biodegradative microbes whereas a decline in the level of sensitive bacteria will result. We propose using the state-of-the-art metagenomic approach to determine the changes in the microbial community composition; and PCR methodology for monitoring the microbial signature biodegradative genes in the community DNA. Prior to the Deepwater Horozon oil spill, human-induced significant perturbation of the microbial community composition and the nature of the biodegradative microbes and their genes in the coastal sediments of GoM have been virtually unknown. We expect the proposed research will provide a baseline data and understanding of the changes in a well-balanced and stable natural microbial ecosystem in the coastal GoM sediments. In addition, the analysis of the composition of the biodegradative microbial population and their biodegradative genes will help us understand the relevance of these microorganisms in the GoM ecosystem and should another unintended release of crude oil occurs in the GoM, perhaps an effective bioremediation protocol can be implemented by biostimulation of their metabolic potential.